1. Field of the Invention
The present invention relates to an image forming apparatus, and in particular, relates to a configuration to correct skew-feeding and positional deviation in the width direction of a sheet.
2. Description of the Related Art
In the related art, there has been an image forming apparatus such as a copying machine, a printer and a facsimile machine including an image forming portion, a transfer portion to transfer a toner image formed on the image forming portion to a sheet, and a sheet conveying apparatus to convey a sheet to the transfer portion. Some sheet conveying apparatuses include a skew-feeding correction portion to detect sheet skew-feeding quantity and to correct skew-feeding by varying a rotation ratio or a rotation speed of two pairs of conveying rollers arranged in the width direction being perpendicular to the sheet conveying direction. Further, some sheet conveying apparatuses include a lateral-registration correction portion to detect lateral-registration being positional deviation in the width direction of a sheet during conveyance with a sensor until the sheet is conveyed to the transfer portion and to correct the lateral deviation of the sheet by shifting, in the width direction, a pair of conveying rollers to convey the sheet in a state of nipping the sheet. In such a sheet conveying apparatus, the sheet lateral deviation is corrected when conveying the sheet to the transfer portion by shifting the sheet in the width direction with a lateral-registration correction portion after firstly correcting sheet skew-feeding with a skew-feeding correction portion as disclosed in US2002/0017755A1.
In such a sheet conveying apparatus in the related art, when correcting sheet lateral-registration, the pair of conveying rollers is to be on standby at a predetermined position and the pair of conveying rollers is moved in the axial direction corresponding to the sheet deviation quantity after nipping the sheet at the standby position. In this manner, the lateral deviation is corrected. FIGS. 15A to 15D are views illustrating the configuration of the lateral-registration correction portion of the sheet conveying apparatus in the related art. A pair of conveying rollers 305 capable of shifting in the axial direction (i.e., the width direction) and sheets Sa and Sb are illustrated in FIGS. 15A to 15D. For example, the sheet Sa has length of approximate 330 mm in the width direction corresponding to a larger size than a normal A4 or A3 size. The sheet Sb has length of approximate 182 mm in the width direction as being a sheet of B5R size, for example.
Since it is required that the pair of conveying rollers 305 is capable of performing lateral-shift operation as nipping the sheet regardless of the sheet size, the pair of conveying rollers 305 is arranged to be matched to the small-sized sheet Sb as a result. For example, as illustrated in FIGS. 15A to 15D, the pair of conveying rollers 305 is arranged at a position being apart from the center of the sheets Sa and Sb in the width direction by distance x. With this arrangement, in the case of correcting lateral deviation of the sheets Sa and Sb, the pair of conveying rollers 305 is capable of being shifted in the direction of an arrow respectively in a state of nipping at a position being apart from the center of the sheets Sa and Sb in the width direction by distance x. Accordingly, the lateral deviation of the sheets Sa and Sb can be corrected regardless of the sheet size.
Here, in the sheet conveying apparatus in the related art having such a lateral-registration correction portion, when deterioration with time such as wearing occurs at one pair of the conveying rollers 305, imbalance of friction force occurs and lateral-shift force is imbalanced. When the lateral-shift force is imbalanced, slippage occurs between the pair of conveying rollers 305 and the respective sheets Sa, Sb. Accordingly, when the respective sheets Sa and Sb are moved, skew-feeding occurs. Similarly, the lateral-shift force is imbalanced due to variation of friction force depending on sheet material, so that slippage occurs between the pair of conveying rollers 305 and the respective sheets Sa and Sb and skew-feeding occurs with sheet moving.
To address the above issues, the present invention provides an image forming apparatus capable of preventing skew-feeding occurrence accompanied with lateral deviation correction of a sheet.